As an example of various mass spectrometric methods, there is an ion trap mass spectrometric method. The basic principle of a quadruple ion trap mass spectrometric method is known (Patent Document 1). In the ion trap method, an RF voltage having a frequency of about 1 MHz is applied to a ring electrode to accumulate ions. In an ion trap, ions having above a certain mass become in a stable condition to be accumulated. The lower voltage applied to the ring electrode is swept to the higher one. In this case, the ions having a low mass are first ejected to obtain a mass spectrum.
In this method, however, different kinds of ions having the same mass cannot be discriminated. To improve this, tandem mass spectrometry in an ion trap has been developed. As an example of the tandem mass spectrometry in a quadruple ion trap, collisional activated dissociation with a bath gas in a quadruple ion trap is known (Patent Document 2). In this method, ions generated by an ion source are accumulated into an ion trap to isolate precursor ions having a mass to be detected. After ion isolation, a supplemental AC electric field resonant with the precursor ions is applied between endcap electrodes to expand an ion trajectory. The ions are collided with a bas gas filled in the ion trap to dissociate the ions for detection. Fragment ions exhibit a specific pattern by a difference in molecular structure. Different kinds of ions having the same mass can be discriminated.
To dissociate the ions, an ion trapping potential produced by the voltage applied to the ring electrode must be increased. To increase the trapping potential, the voltage applied to the ring electrode must be set to a high voltage. This moves the fragment ions having a low mass away from the stable trajectory condition so that the ions cannot be trapped. In matrix-assisted laser desorption ionization, monovalent ions having a high mass (molecular weight above 2000) are easily generated. These are stable in structure to be hard to dissociate in the general collisional activated dissociation.
There is known a method for solving the problems that fragment ions having a low mass cannot be detected and that ions having a high mass cannot be dissociated (Non-Patent Document 1). In this method, in addition to a continuously-introduced bath gas (Typically, He is used.) continuously introduced for cooling ions, a gas having a large molecular weight such as Ar is intermittently introduced from a gap between endcap electrodes and a ring electrode using a switchable solenoid valve to promote ion collisional activated dissociation in an ion trap. After these operations, an RF voltage applied to the ring electrode is increased to sequentially eject ions from the ion trap for detection. This can increase the ion excitation effect in the ion trap to detect fragment ions having a lower mass. A method for applying the same method as this method to an ion trap-hybrid time-of-flight mass spectrometer is known (Non-Patent Document 2). Also in this method, in addition to a general continuously-introduced bath gas (Typically, He is used.), a gas having a large molecular weight such as Ar is intermittently introduced from a gap between endcap electrodes and a ring electrode using a high-speed switchable solenoid valve to promote ion collisional activated dissociation in an ion trap. After these operations, a DC voltage is applied to the endcap electrode and the ring electrode to draw ions for coaxial acceleration, thereby performing mass dissociation from the time of flight of the ions. This can detect fragment ions having a lower mass at high mass accuracy.
There is known a method for solving the general collisional activated dissociation problems that fragment ions having a low mass cannot be detected and that ions having a high mass cannot be dissociated (Non-Patent Document 3). In this method, after ion isolation, a CO2 laser is irradiated from a hole through a ring electrode onto the center part of a trap. The ions absorb an infrared ray to advance dissociation by excitation of an internal energy. In this method, a quadruple ion trap mass spectrometer can detect fragment ions having a low mass.
[Patent Document 1]                U.S. Pat. No. 4,650,999        
[Patent Document 2]                U.S. Pat. No. 4,736,101        
[Non-Patent Document 1]                Richard W. Vachet and Gray L. Glish, Journal of American Society for Mass Spectrometry, Vol. 7, pp. 1194-1202, 1996)        
[Non-Patent Document 2]                Li Ding et al., Proceeding SPIE the international Society for Optical Engineering, 1999, Vol. 3777, pp. 144-155        
[Non-Patent Document 3]                Armando Colorado et al., Analytical Chemistry, 1996, Vol. 68, pp. 4033-4043        